I started this game with my first daughter when she was about 18-months-old. It worked so quickly that I was shocked, but hey when you do something right, be excited. We got complements constantly for what a smart child we had, and of course we could only agree. I will share my secret with you so that your small child can impress too with their knowledge of shapes and colors.
Once they have this basic step down make it harder by not showing them your set. Just ask them to find the shape you want them to find.